1. Field of the Invention
The present invention relates to a fuel cell system for warming up a fuel cell by combusting an anode off-gas.
2. Description of the Related Art
These years a fuel cell electric vehicle (FCEV) is noticed from a viewpoint of suppressing a discharge amount of carbon dioxide causing global warming and the like. The FCEV mounts a fuel cell (FC) for generating power by electrochemically reacting hydrogen (H2) and oxygen (O2), supplies electric power generated by the fuel cell to a traction motor, and produces driving force.
In this connection, the fuel cell brings out its performance at a certain temperature. For example, in a proton exchange membrane (PEM) fuel cell the temperature is about 80 degrees Celsius, and if the temperature is lower, a power generation performance (electromotive force) is lowered. Therefore, when activating an FECV in winter or a cold district, it is necessary to warm up the fuel cell (that is, to heat up or warm the fuel cell to a predetermined temperature).
Conventionally, in warming up the fuel cell are taken following methods: For example, there is a method of additionally providing the fuel cell with an electric heater, making the electric heater generate heat by a battery mounted on the FCEV, and warming up the fuel cell by the heat (for example, see a paragraph 0038 and FIG. 3 of Japanese Patent Laid-Open Publication Hei. 7-94202 (hereinafter referred to as patent document 1)). In addition, there is another method of additionally providing the fuel cell with a combustion heater (catalyst combustor and the like), supplying hydrogen, methanol, gasoline, and the like mounted as fuel by the FCEV to the combustion heater, combusting them, and warming up the fuel cell by combustion heat thereof (for example, see a paragraph 0034 and FIG. 1 of Japanese Patent Laid-Open Publication No. 2001-118593 (hereinafter referred to as patent document 2)).
On the other hand, in a fuel cell equipped with a fuel cell circulation path for recirculating a fuel gas out of fuel cells, because water produced at a cathode moves to an anode through a membrane together with power generation, a flow passage of the anode is occluded by water, and similarly a nitrogen gas and the like move from the cathode to the anode through the membrane, it cannot be avoided that impurities are accumulated in the fuel cell circulation path. Therefore, in order to stably generate power by this kind of a fuel cell, there is a fuel cell that discharges the fuel gas of the fuel cell circulation path as an anode off-gas from a discharge side path (that is, performs purging and sweeping), depending on a state within the fuel cell, mixes and dilutes the fuel gas with a cathode off-gas, then combusts it by a catalyst combustor, and discharges it in an atmosphere (for example, see paragraphs 0063 to 0065 and FIG. 1 of Japanese Patent Laid-Open Publication No. 2002-289237 (hereinafter referred to as patent document 3)). In addition, there is also a fuel cell that combusts reaction-remaining hydrogen discharged from the anode of the fuel cell and a hydrogen gas from a hydrogen tank together with air by a catalyst combustor and performs a warm-up by supplying a combustion heat thereof to the fuel cell through a coolant circulation path (for example, see paragraphs 0022 to 0024 and FIG. 1 of Japanese Patent Laid-Open Publication No. 2003-243009 (hereinafter referred to as patent document 4)).
However, in an apparatus of the patent document 1 it is not preferable to consume electric power of a vehicle-mounted battery for an electric heater for a warm-up, and moreover, there is a case that sufficient electric power is not supplied from the vehicle-mounted battery in a low temperature when electromotive force becomes lower. In addition, when using a commercial power source, there is a problem that a warm-up cannot be performed at a place such as a road where there is no commercial power source. In an apparatus of the patent document 2, because a hydrogen combustor and the like consume hydrogen (raw fuel) used for power generation, there is a problem that a fuel consumption of the fuel cell results in being exacerbated.
On the other hand, in an apparatus of the patent document 3, although a hydrogen concentration of an anode off-gas can be reduced to a safe level by a dilution and a combustion and the anode off-gas can be discharged, there is a problem that an energy efficiency lowers because a thermal energy produced by the combustion of a catalyst combustor is discharged in an atmosphere and a warm-up of the fuel cell has to be performed by a heating mechanism such as an electric heater. In addition, in an apparatus of the patent document 4, when an introduction hydrogen amount into a catalyst combustor is limited, a needed amount of reaction-remaining hydrogen results in not being discharged, an operation state of the fuel cell system becomes unstable, and there is a possibility that a lowering of a power output and the like occur. In addition, an excess of humidifying water and a moisture reversely diffusing into the anode from the cathode mix with the reaction-remaining hydrogen, these moistures are introduced in a liquid state, and thereby there is also a possibility that a combustion in a catalyst combustor results in becoming unstable.
Consequently, a fuel cell system is strongly requested that performs a speedy warm-up by an anode off-gas without obstructing a stable operation of the fuel cell system.